Promethazine is a first generation antihistamine. It can be used to treat symptoms of allergies, motion sickness and insomnia and is sometimes used as an anti-emetic. It is available in oral, injectable and rectal preparations. In some countries a prescription from a doctor is required, while in others it is available as an over-the-counter preparation.

The symptoms of allergies include runny nose, sneezing, streaming eyes and itching due to a skin reaction. This is caused mainly by the release of histamine in response to allergens such as pollen, animal hair, medication or food to which the patient is allergic. Promethazine works to negate these effects by blocking the histamine receptors. It is a potent H1 receptor antagonist, and this is its main mechanism of action.

In severe cases of allergy, the chest may close up due to increased bronchial secretions. This is a medical emergency and requires urgent medical attention. Promethazine may be used in this acute state at the hospital, as part of treatment. Patients with known allergies should wear a bracelet that states their allergies, and should keep an antihistamine like promethazine with them at all times.

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Promethazine can cause drowsiness, so driving or working with heavy machinery while taking it should be avoided. This effect is used therapeutically in some cases to treat insomnia and as an adjunctive sedative and anxiolytic agent before surgery. Due to its sedative effects, it should not be taken in combination with other central nervous system (CNS) drugs or alcohol.

It also has anti-emetic properties and may be used to treat motion sickness. As with any drug, the use of promethazine for any of these indications should be discussed with a medical practitioner. They will take into consideration the patient's medical history, concomitant medicines and other factors, such as pregnancy, desired pregnancy, or lactation.

Promethazine should not be used in children under two years old and should be used with caution in children over two years old, and only under medical supervision. Care should be taken when administering combination products such as cold and flu preparations, as some may contain promethazine as a constituent. There have been reports of respiratory depression and even death, possibly due to promethazine use in small children.

The dose and duration of therapy is determined by the indication. Some chronic allergy sufferers require long-term antihistamine throughout the allergy season. In the case of pre-operative sedation, a single dose may be used. For motion sickness, the drug should be taken only while traveling. The correct dosage and duration can be determined by the medical practitioner.

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